This invention relates to an iron-phosphorus electroplating bath from which crack-free iron-phosphorus films can be electroplated.
Electroplated iron-phosphorus films have a higher hardness than electroplated iron films. It is thus expected that slide members such as pistons can be improved in abrasion resistance and galling resistance by forming a plated iron-phosphorus film on the necessary portion of slide members, for example, the skirt of pistons.
Prior art known iron-phosphorus electroplating baths are those comprising a ferrous ion, hypophosphorous acid or a hypophosphite, and optionally, boric acid or ammonium chloride. As long as we have confirmed, electroplating in the conventional iron-phosphorus electroplating baths results in iron-phosphorus films which develop many cracks in their cross section. The occurrence of cracks becomes a bar in applications requiring improved mechanical performance. An iron-phosphorus film electroplated on a workpiece and having cracks developed therein not only displays a remarkably reduced toughness in itself, but also tends to reduce the toughness of the workpiece due to the wedge or notch effect.